1. Field of the Invention
The present invention relates to EL display apparatus for displaying images or the like using an electroluminescent (hereinafter abbreviated as xe2x80x9cELxe2x80x9d) layer.
2. Description of the Related Art
Attention has been paid for years to EL display apparatus of a basic configuration as shown in FIG. 15 as a self-luminescent type flat display apparatus. The basic configuration of an EL display apparatus includes an electrically insulating transparent substrate 1 such as made of glass, strip-shaped transparent electrodes 2 arranged as extending parallel with each other on the glass substrate 1, a dielectric material layer 3 covering the transparent electrodes 2, and an EL layer of an inorganic material formed on the dielectric material layer 3. On this structure are further stacked a dielectric material layer 5 and strip-shaped back face electrodes 6 arranged as extending parallel with each other on the dielectric material layer 5. The transparent electrodes 2 and the back face electrodes 6 are each formed as a group of parallel fine wires and extend perpendicularly to each other.
FIG. 16 shows an applied voltage-luminance characteristic of the EL layer 4 shown in FIG. 15. The EL layer 4 is formed by doping zinc sulfide (ZnS) as an inorganic fluorescent substance with an activator such as manganese (Mn). When the applied voltage reaches about 180 V, luminescence begins to occur and its luminance grows higher with rising voltage. When a voltage higher than a given voltage, for example about 230 V, is applied, the EL layer 4 gives off light at a sufficient luminance. Thus, the EL layer 4 needs to be driven by application of a relatively high voltage of about 200 V for its luminescence to occur.
EL display apparatus of the type shown in FIG. 15 are adapted to display an image on the glass substrate 1 side. In this case, the transparent electrodes 2 on the glass substrate 1 side serve as data side electrodes, while the back face electrodes 6 serves as scanning side electrodes. The points of intersection of the transparent electrodes 2 on the data side and the back face electrodes 6 on the scanning side each form a pixel. Thus, a display panel has a plurality of such pixels arranged in a matrix pattern.
FIG. 17 shows a change in characteristics with time in the case where each electrode is driven by application of a symmetric waveform and that in the case where each electrode is driven by application of an a symmetric waveform for comparison. As seen from the characteristic resulting from the application of the asymmetric waveform plotted by one-dot chain line, the luminance varies in a direction such as to become lower than the initial characteristic. Application of a symmetric waveform causes a change plotted by the two-dot chain line to occur and hence can avoid such deterioration in performance involved in the case of application of an asymmetric waveform.
FIG. 18 schematically illustrates an electric circuit configuration of a driving circuit of a conventional EL display apparatus. The driving circuit configuration shown is basically equivalent to that disclosed in Japanese Examined Patent Publication JP-B2 6-34152 (1994) invented by Applicant of the instant application. Such a driving circuit provided in an EL display panel of the structure shown in FIG. 15 includes a data side driving IC 11 as a semiconductor integrated circuit for driving the transparent electrodes 2 as data side electrodes, and a scanning side driving IC 12 for driving back face electrodes 6 as scanning side electrodes. The data side driving IC 11 incorporates a shift register latch 13, and switching devices each comprising a pull-up element 14 and a pull-down element 15. Similarly, the scanning side driving IC 12 incorporates a shift register latch 16, and switching devices each comprising a pull-up element 17 and a pull-down element 18. The pull-up elements 14 and pull-down elements 15 of the data side driving IC 11 form an output circuit adapted to drive data electrodes 19. The pull-up elements 17 and pull-down elements 18 of the scanning side driving IC 12 form a driving circuit adapted to drive scanning electrodes 20.
The source electrodes of the pull-up elements 14 in the data side driving IC 11 are connected to a common line and applied with a positive modulating voltage of +Vm. Similarly, the source electrodes of the pull-down elements 15 are connected to a common line and are grounded to assume a GND potential. The drain electrode of each pair of pull-up element 14 and pull-down element 15 is connected to each data electrode 19. The data electrodes 19 consist of, for example, n electrodes X1, X2, X3, . . . , Xn, each of which are applied with a modulating voltage of Vm by the associated pull-up element 14 or pull-down element 15 in response to an image signal. The source electrodes of the pull-up elements 17 in the scanning side driving IC 12 are connected to a common line and supplied with a high positive voltage from a positive voltage supply circuit 21, which is also indicated at Pd1. The source electrodes of the pull-down elements 18 are connected to a common line and supplied with a high negative voltage from a negative voltage supply circuit 22, which is also indicated at Nd1. A diode 23 is provided between the positive voltage supply circuit 21 and the source electrodes of the pull-up elements 17. A diode 24 is provided between the negative voltage supply circuit 22 and the source electrodes of the pull-down elements 18. The diode 23 has an anode side connected to the positive voltage supply circuit 21 and a cathode side connected to the source electrodes of the pull-up elements 17. The cathode side of the diode 23 is also connected to the cathode side of a diode 25, and the anode side of the diode 25 is connected to the ground potential GND through a switching circuit 28 (Nd2). The diode 24 has an anode side connected to the source electrodes of the pull-down elements 18 and a cathode side connected to the negative voltage supply circuit 22. The anode side of the diode 24 is also connected to the anode side of a diode 26, and the cathode side of the diode 26 is connected to the ground potential GND through a switching circuit 27 (Pd2).
FIG. 19 schematically illustrates the configuration of a power supply circuit for generating a write voltage, which is a high voltage required to drive the EL display panel 10 shown in FIG. 18. A switching device 30 causes current on the primary side of a transformer 31 to be interrupted or passed. A pair of windings are provided on the secondary side of the transformer 31, and diodes 32 and 33 and capacitors 34 and 35 associated with the windings provide a smoothed positive voltage of +Vw and a smoothed negative voltage of xe2x88x92Vw. An output of the positive side diode 32 on its cathode side associated with the capacitor 34 is supplied to the anode of a diode 36, and the cathode side of the diode 36 is connected to the output side of transistors 39 and 40 controlled by a control circuit 38. When the transistors 39 and 40 are operated by the control circuit 38 so that a positive modulating voltage of +Vm is obtained from the output side thereof, an output of Vw+Vm as a positive write voltage is obtained on the cathode side of the diode 36.
The voltages of Vm and Vw, respectively, are established within the following ranges. Vm is a voltage for controlling the occurrence of luminescence of the EL display panel 10 and can assume any predetermined value lower than the luminescence initiating voltage. Vw is established to assume a value such that the sum of Vw and Vm is higher than the luminescence initiating voltage of the EL display panel 10 thereby ensuring a sufficient luminescence intensity.
FIG. 20 shows driving waveforms at different portions of data electrodes and scanning electrodes driven by the circuit shown in FIG. 18 in the case where the EL element on the point of intersection (X1,Y1) of X1 selected from the data electrodes 19 and Y1 selected from the scanning electrodes 20 is caused to give off light, while the EL element on the point of intersection (X2, Y1) of X2 selected from the data electrodes 19 and Y1 of the scanning electrodes 20 is not caused to give off light. In this case, what is called xe2x80x9cframe reversal drivexe2x80x9d is performed to apply well-symmetrized alternating pulses to the EL layer thereby realizing highly reliable display.
In the first frame, first, display data (Data) and clock (CkD) are sequentially inputted to the data side driving IC 11 and then transferred to a specified data electrode 19 with use of the shift register latch 13, followed by temporary latching of the display data using a latch strobe (LS). What is represented by N/R is an input terminal for specifying a direction in which display data is to be shifted. With the scanning electrodes 20 connected to the scanning side driving IC 12 being kept at a floating potential, pull-down elements 15 associated with those data electrodes 19 including X1 on which EL devices intended for luminescence lie are turned ON so that these associated data electrodes 19 assume the GND potential while pull-up elements 14 associated with those data electrodes 19 including X2 on which EL devices not intended for luminescence lie are turned ON to charge these associated data electrodes 19 up to a voltage of +Vm according to the data latched by the shift register latch 13 of the data side driving IC 11.
In turn, the pull-up element 17 of the scanning side driving IC 12 connected to Y1 of selected scanning electrodes 20 inputs a voltage of +(Vw+Vm) supplied from the positive voltage supply circuit 21 to the scanning electrode Y1 to raise the potential of Y1 up to +(Vw+Vm). As a result, the EL device on the point of intersection (X1,Y1) of the data electrode X1 and the scanning electrode Y1 is applied with a voltage of +(Vw+Vm), which is sufficient to cause luminescence, and hence gives off light. On the other hand, the EL device on the point of intersection (X2,Y1) of the data electrode X2 and the scanning electrode Y1 is applied with a voltage of Vw, which is insufficient to cause luminescence, and hence does not give off light.
Subsequently, all the data electrodes 19 (X1 through Xn) connected to the data side driving IC 11 are discharged to the ground potential GND by turning the pull-down elements 15 ON. Further, electric charge accumulated on the selected scanning electrode Y1 is discharged by means of the pull-down element 18 connected thereto and the switching circuit 27 so that the scanning electrode Y1 assume the ground potential GND. Thus, the driving operation with respect to the selected scanning electrode Y1 ends.
A similar driving operation is repeated with respect to scanning electrodes Y1 to Ym sequentially line by line to complete the driving operation in the first frame.
In the subsequent second frame, as in the first frame, display data (Data) and clock (CkD) are sequentially inputted to the data side driving IC 11 and then transferred to a specified location with use of the shift register latch 13, followed by temporary latching of the display data. With the scanning electrodes 20 connected to the scanning side driving IC 12 being kept at a floating potential, pull-up elements 14 associated with those data electrodes 19 including X1 on which EL devices intended for luminescence lie are turned ON so that these data electrodes 19 are charged up to a potential of +Vm while pull-down elements 15 associated with those data electrodes 19 including X2 on which EL devices not intended for luminescence lie are turned ON to have these data electrodes 19 assume the GND potential.
In turn, the pull-down element 18 connected to Y1 selected from the scanning electrodes 20 uses a voltage of xe2x88x92(Vw) supplied from the negative voltage supply circuit 22 to lower the potential of Y1 to xe2x88x92(Vw). As a result, the EL device on the point of intersection (X1,Y1) of the data electrode X1 and the scanning electrode Y1 is applied with a voltage of xe2x88x92(Vw+Vm), which is sufficient to cause luminescence, and hence gives off light. On the other hand, the EL device on the point of intersection (X2,Y1) of the data electrode X2 and the scanning electrode Y1 is applied with a voltage of Vw, which is insufficient to cause luminescence, and hence does not give off light. Subsequently, the pull-down elements 15 connected to all the data electrodes 19 (X1 through Xn) are turned ON to discharge the data electrodes 19 down to the GND potential. Further, the selected scanning electrode Y1 is discharged down to the GND potential by means of the pull-up element 17 connected thereto and the switching circuit 28. Thus, the driving operation with respect to the selected scanning electrode Y1 ends.
A similar driving operation is repeated with respect to scanning electrodes Y1 to Ym sequentially line by line to complete the driving operation in the second frame. Alternating the first frame operation and the second frame operation makes it possible to apply positive and negative alternating pulses to the EL display panel 10 thereby displaying a desired image.
Japanese Examined Patent Publication JP-B2 2619001 discloses a method of correcting a change in luminance depending on the number of pixels giving off light and the gray scale level in a gray scale display by varying a modulating voltage or adjusting the time period for which a modulating voltage is applied, or by any other means in accordance with gray scale data. According to this prior art reference, the modulating voltage to be applied to a data electrode is varied based on the sum total of luminescent loads found from the gray scale data.
FIG. 21 shows driving waveforms used in the gray scale display disclosed in JP-B2 2619001. In this prior art technique, the time period for which a capacitor is charged is determined in accordance with the gray scale data for each pixel and, therefore, the capacitor is more charged to provide a higher write voltage as the number of pixels intended for luminescence increases to provide a brighter gray scale level. Thus, it is possible to correct the luminance in accordance with an increase in load.
In the driving circuit of the conventional EL display panel 10 shown in FIG. 18, the GND potential and the positive voltage of +Vm are used as modulating voltages to be applied from the data side through the data side driving IC 11 and, hence, the waveforms of the voltages applied to the EL display panel 10 are not symmetric. In order to correct such a symmetric waveforms into symmetric waveforms, the absolute value of amplitude of a driving voltage to be applied to a data electrode 19 from the scanning side driving IC 12 needs to be changed depending upon whether the voltage to be applied to the data electrode 19 is of positive or negative polarity. In FIG. 20, for instance, a positive voltage having an amplitude of Vw+Vm is applied to the scanning electrode Y1 in the first frame, whereas a negative voltage having an amplitude of xe2x88x92Vw is applied thereto in the second frame. This results in the design of the power supply circuit made intricate or a like problem. Further, the withstand voltage of a decoupling part connected to the power supply line and that of a bypass capacitor need to be changed depending upon whether they are on the positive side or the negative side. This results in a difficulty in the management of parts or an increase in the cost of parts if they are rendered common based on a voltage amplitude of a higher absolute value.
The aforementioned correction in a gray scale display is performed to correct a change in the voltage applied to a pixel intended for luminescence light due to influences of the ON resistance of an output device, wiring resistance of electrodes or the like that vary in accordance with the number of pixels giving off light on each scanning side electrode and the level of gray scale. The correction is achieved by increasing or decreasing the write voltage of positive or negative polarity. For this reason, it is possible that the symmetry of applied voltages is impaired. If the symmetry of applied voltages is impaired due to the gray scale correction, a change in the applied voltage-luminance characteristic is likely to occur as shown in FIG. 16 after the apparatus has been driven for a long time and such a change causes the display quality to be degraded. For this reason, such a gray scale correction cannot be said to be a desirable correction from the viewpoint of long-term reliability.
An object of the invention is to provide an electroluminescent display apparatus having simplified peripheral circuitry which is driven with well-symmetrized driving waveforms and imparted with higher long-term reliability.
The invention provides an electroluminescent display apparatus comprising:
a first group of electrodes;
a second group of electrodes,
the first group of electrodes and second group of electrodes being arranged to extend in respective directions intersecting each other;
an electroluminescent layer sandwiched between the first group of electrodes and the second group of electrodes;
a first driving circuit, connected to the first group of electrodes, for applying a modulating voltage of positive or negative polarity to the first group of electrodes through output terminals thereof; and
a second driving circuit, connected to the second group of electrodes, for applying write voltages of positive or negative polarity, having an equal absolute value, to the second group of electrodes through output terminals thereof, or capable of switching a potential of the second group of electrodes to either a ground potential or a floating potential.
According to the invention, the second driving circuit applies a positive write voltage or a negative write voltage which have an equal absolute value to the second group of electrodes and, hence, a simplified power supply circuit for supplying such write voltages is realized with the result that the peripheral circuitry can be simplified.
In the invention it is preferable that the absolute value of the write voltage is selected to be larger than that of a voltage that initiates luminescence of the electroluminescent layer and lower than a luminescence voltage at which the electroluminescent layer is in a luminance saturation zone; and
the modulating voltage is of such a magnitude that a sum of the modulating voltage and the write voltage increases up to the luminescence voltage within the luminance saturation zone while a difference obtained by subtracting the modulating voltage from the write voltage decreases from the luminescence initiating voltage to a voltage within a predetermined range.
According to the invention, the absolute value of the write voltage is selected so as to be larger than that of the voltage that initiates luminescence of the electroluminescent layer and lower than a luminescence voltage at which the electroluminescent layer is in a luminance saturation zone and, hence, the write voltage is higher than the modulating voltage. Since the positive and negative write voltages which are on the higher voltage side are well-symmetrized, it is possible to use capacitors or like components having a common withstand voltage for both the positive and negative polarities of the write voltage in the peripheral circuitry. Further, since a maximum value of the write voltage can be decreased as compared with that of an asymmetric write voltage, the required withstand voltage rank is likely to lower. Such a lowered withstand voltage rank makes reductions in cost and size possible.
In the invention it is preferable that the write voltage is selected to be a mid-voltage between the luminescence initiating voltage and the luminescence voltage, and the modulating voltage is selected so that a maximum value thereof is xc2xd as large as the difference between the luminescence initiating voltage and the luminescence voltage.
According to the invention, the luminance of luminescence can be raised by increasing the sum of the write voltage and the modulating voltage as a voltage to be applied to the electroluminescent layer, to the luminescence voltage, or the luminance can be lowered by decreasing the difference obtained by subtracting the modulating voltage from the write voltage, to around the luminescence initiating voltage. Further, the maximum value of the modulating voltage can be minimized in a range in which a change in luminance is larger, whereby the peripheral circuitry of the first driving circuit and the like can be simplified.
In the invention it is preferable that the first driving circuit is capable of varying the modulating voltage according to a signal inputted there to and outputting the modulating voltage thus varied.
According to the invention, a voltage to be outputted as the modulating voltage from the first driving circuit can be varied according to a signal inputted to the driving circuit, whereby a gray scale display can be achieved easily.
The invention provides an electroluminescent display apparatus comprising:
a first group of electrodes;
a second group of electrodes,
the first group of electrodes and second group of electrodes being arranged to extend in respective directions intersecting each other;
an electroluminescent layer sandwiched between the first group of electrodes and the second group of electrodes,
points of intersection of the first group of electrodes and the second group of electrodes being driven with pulse waveforms to serve as pixels and display a gray scale image;
a first driving circuit having output terminals connected to respective ones of the first group of electrodes and capable of applying a modulating voltage of positive or negative polarity to each of the first group of electrodes;
a second driving circuit having output terminals connected to respective ones of the second group of electrodes and capable of switching each of the second group of electrodes between a state applied with a write voltage of positive or negative polarity and a state applied with a ground potential or a floating potential; and
a correction circuit for computing display data indicative of a gray scale level for each of the pixels formed on the second group of electrodes line by line in the direction in which the second group of electrodes are arranged and varying a pulse width of a voltage waveform to be applied to each of the pixels on each line in accordance with the display data computed.
According to the invention, the first group of electrodes and the second group of electrodes, which are arranged to extend in respective directions which intersect each other and sandwich the electroluminescent layer therebetween, are driven by the first driving circuit and the second driving circuit, respectively. Each of the first group of electrodes is driven by being applied with a modulating voltage of positive or negative polarity by the first driving circuit. The second driving circuit is capable of switching each of the second group of electrodes between a state applied with a write voltage of positive or negative polarity and a state applied with a ground potential or a floating potential. Since the positive and negative voltages applied to each pixel by the first and second driving circuits have an equal absolute value, the symmetry of the voltages with respect to the polarity can be maintained. The correction circuit is configured to compute display data indicative of a gray scale level for each of the pixels formed on the second group of electrodes line by line in the direction in which the second group of electrodes are arranged and to vary a pulse width of a voltage waveform to be applied to each of pixels on each line in accordance with the display data computed. Accordingly, it is possible to realize a display with an even luminance regardless of a variation in the number of pixels giving off light by increasing the pulse width when the number of such pixels is large or decreasing the pulse width when the number of such pixels is small even when such pixels are at the same gray scale level.
In the invention it is preferable that the correction circuit is configured to vary the pulse width of the voltage waveform of at least one of the modulating voltage to be applied from the first driving circuit and the write voltage to be applied from the second driving circuit.
According to the invention, since the pulse width of the voltage waveform of at least one of the modulating voltage to be applied from the first driving circuit and the write voltage to be applied from the second driving circuit can be varied, the pulse width of the voltage waveform applied to each pixel also can be varied thereby enabling a correction for even luminance according to an increase or decrease in load.
In the invention it is preferable that the correction circuit is configured to vary relative timing between the modulating voltage to be applied from the first driving circuit and the write voltage to be applied from the second driving circuit.
According to the invention, the correction circuit varies relative timing between the modulating voltage to be applied from the first driving circuit and the write voltage to be applied from the second driving circuit and, hence, an overlap of the waveform of the modulating voltage and the waveform of the write voltage is varied with respect to time in applying the modulating voltage and the write voltage to each pixel and such a variation is equivalent to a variation in the pulse width of a voltage waveform for driving the pixel. Thus, a correction for an even luminance accommodating a variation in load can be achieved.
In the invention it is preferable that the correction circuit is configured to vary the pulse width of a voltage waveform to be applied to each of the pixels on each line equally with respect to positive polarity and negative polarity.
According to this feature of the invention, the correction circuit varies the pulse width of a positive voltage waveform and that of a negative voltage waveform to be applied to each pixel equally and, hence, each pixel can be driven equally and symmetrically on the positive and negative sides in terms of not only voltage but also time, whereby the long-term reliability of the apparatus can be enhanced.
In the invention it is preferable that the correction circuit is configured to compute all or part of the gray scale data and vary the pulse width of a voltage waveform to be applied to each pixel according to all or part of the gray scale data computed.
According to this feature of the invention, the correction circuit computes all or part of gray scale data and varies the pulse width of a voltage waveform to be applied to each pixel according to all or part of the gray scale data computed, thereby achieving a simplified correction.